1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film with the use of laser light (hereinafter referred to as laser annealing) and to a laser apparatus for performing the laser annealing (an apparatus including a laser and an optical system for leading laser light output from the laser to a process object). The invention also relates to a semiconductor device fabricated by a manufacturing process that comprises the laser annealing step, and to the manufacturing process. The semiconductor device here includes an electro-optical device such as a liquid crystal display device and an EL display device, and an electronic device having the electro-optical device as one of its components.
2. Description of the Related Art
An advance has been made in recent years in development of thin film transistors (hereinafter referred to as TFTs), and TFTs using polycrystalline silicon films (polysilicon films) as crystalline semiconductor films are receiving the attention. In liquid crystal display devices (liquid crystal displays) and EL (electroluminescence) display devices (EL displays), in particular, such TFTs are used as elements for switching pixels and elements for forming driver circuits to control the pixels.
General means for obtaining a polysilicon film is a technique in which an amorphous silicon film is crystallized into a polysilicon film. A method in which an amorphous silicon film is crystallized with the use of laser light has lately become the one that is especially notable. In this specification, to crystallize an amorphous semiconductor film with laser light to obtain a crystalline semiconductor film is called laser crystallization.
The laser crystallization is capable of instantaneous heating of semiconductor film, and hence is an effective technique as measures for annealing a semiconductor film formed on a low heat resistant substrate such as a glass substrate or a plastic substrate. In addition, the laser annealing makes the throughput definitely higher as compared with conventional heating measures using an electric furnace (hereinafter referred to as furnace annealing).
There are various kinds of laser light, of which the general one to be used in laser crystallization is laser light generated and emitted from a pulse oscillation type excimer laser as a source (hereinafter referred to as excimer laser light). The excimer laser has advantages in that it is large in output and that it is capable of repetitive irradiation at a high frequency and, moreover, excimer laser light is advantageous in terms of its high absorption coefficient with respect to silicon films.
To generate excimer laser light, KrF (wavelength, 248 nm) or XeCl (wavelength, 308 nm) is used as an excitation gas. However, Kr (krypton) gas and Xe (xenon) gas are very expensive, causing a problem of increase in production cost when recharge of the gas is frequent.
In addition, every two or three years, excimer laser annealing requires replacement of attachments such as a laser tube for laser oscillation and a gas refinery for removing unnecessary compounds that are produced during the course of oscillation. Many of these attachments are also expensive, taking part in increasing the production cost.
As seen in the above, a laser apparatus using excimer laser light does possess high ability but also possess drawbacks in that maintenance thereof is very troublesome and that the running cost (which means the costs required for operating the apparatus) is high for a laser apparatus for mass production.
The present invention has been made in view of the above, and an object of the present invention is therefore to provide a laser apparatus which is capable of providing a crystalline semiconductor film with a larger crystal grain size than in prior art and which is low in running cost, and to provide a laser annealing method using that laser apparatus. Another aspect of the present invention is to provide a semiconductor device fabricated by using the laser annealing method and a method of manufacturing the semiconductor device.
The present invention is characterized in that the front side and the back side of a semiconductor film are irradiated with laser light generated and emitted from a solid state laser (a laser that outputs laser light using a crystal rod as a resonance cavity) as a source.
When the semiconductor film is irradiated, the laser light is preferably linearized by an optical system. To linearize laser light indicates that laser is formed into such a shape as to make the irradiated area linear when a process object is irradiated with the laser light. In short, it indicates that the sectional shape of the laser light is linearized. The term xe2x80x9clinearxe2x80x9d here does not mean a line in the strict sense of the word, but means a rectangle (or an oblong) with a large aspect ratio. For instance, a rectangle or an oblong having an aspect ratio of 10 or more (preferably 100 to 10000).
In the above construction, the solid state laser may be generally known ones such as a YAG laser (which usually indicates an Nd:YAG laser), an Nd:YVO4 laser, an Nd:YAIO3 laser, a ruby laser, a Ti:sapphire laser, or a glass laser. The YAG laser is particularly preferable because of its superiority in coherence and pulse energy. There are a continuous wave YAG laser and a pulse oscillation type YAG laser and the latter is desirable in the present invention, for it is capable of large area irradiation.
However, the fundamental wave (a first harmonic) of the YAG laser has as high wavelength as 1064 nm. It is therefore preferable to use second harmonic (wavelength, 532 nm), third harmonic (wavelength, 355 nm), or fourth harmonic (wavelength, 266 nm).
In particular, the second harmonic of the YAG laser has a frequency of 532 nm and is within a wavelength range (around 530 nm) in which reflection at an amorphous silicon film is the least when the amorphous silicon film is irradiated with the second YAG laser wave. In this wavelength range, in addition, the quantity of transmittable laser light through the amorphous semiconductor film is enough to efficiently irradiate again the amorphous semiconductor film from its back side using a reflective member. Moreover, the laser energy of the second harmonic is large, about 1.5 J/pulse at a maximum (in an existing pulse oscillation type YAG laser apparatus). When it is linearized, the length thereof in the longitudinal direction is therefore markedly lengthened to make it possible to irradiate a large area at once with laser light. These harmonics can be obtained by using a non-linear crystal.
The fundamental wave can be modulated into the second harmonic, the third harmonic, or the fourth harmonic by a wavelength modulator that includes a non-linear element. The respective harmonics may be formed by following any known technique. In this specification, xe2x80x9claser light generated and emitted from a solid state laser as a sourcexe2x80x9d includes not only the fundamental wave but also the second harmonic, the third harmonic, and the fourth harmonic which are obtained by modulating the wavelength of the fundamental wave.
Alternatively, the Q switch method (Q modulation switch method) that is often used in the YAG laser may be employed. This method is to sufficiently lower the Q value of a laser resonator in advance and to then rapidly raise the Q value, to thereby output sharp pulse laser having a very high energy value. The method is one of known techniques.
The solid state laser used in the present invention can output laser light as long as a solid crystal, a resonant mirror, and a light source for exciting the solid crystal are satisfied, basically. Therefore, maintenance thereof is not laborious unlike the excimer laser. In other words, the running cost of the solid state laser is significantly less as compared with the excimer laser, making it possible to greatly reduce the production cost of a semiconductor device. A decrease in number of the maintenance leads to an increase of the operating rate of the mass-production line, so that the throughput along the manufacturing steps is improved as a whole. This also contributes considerably to the reduction in production cost of the semiconductor device. Moreover, the solid state laser occupies a smaller area than the excimer laser does, which is advantageous in designing a production line.
In addition, to perform laser annealing by irradiating the front side and the back side of an amorphous semiconductor film with laser light allows obtainment of a crystalline semiconductor film with a larger crystal grain size than in prior art (where the amorphous semiconductor film is irradiated with laser light only from its front side). According to the applicant of the present invention, it is considered that irradiation of laser light onto the front side and the back side of an amorphous semiconductor film slows down the cycle of fusion and solidification of the semiconductor film, and that the crystal grain size is increased as a result.
The obtainment of a crystalline semiconductor film with a large crystal grain size leads to a considerable improvement of the performance of the semiconductor device. taking a TFT as an example, enlargement of a crystal grain size allows reduction in number of crystal grain boundaries that may be contained in a channel formation region. That is, it allows fabricating a TFT that has one, preferably zero, crystal grain boundary in its channel formation region. Since the crystallinity of each crystal grain is such that it may substantially be regarded as a single crystal, to obtain a mobility (electric field effect mobility) equal to or higher than that of a transistor using a single crystal semiconductor is also possible.
Further, carriers cross the crystal grain boundaries extremely less frequently in the present invention to thereby reduce the fluctuation of ON current values (drain current when a TFT is in ON state), OFF current values (drain current when a TFT is in OFF state), threshold voltage, of S values, and electric field effect mobility.